Mobile ad-hoc networks (MANET's) are becoming increasingly popular because they operate as self-configuring networks of mobile routers or associated hosts connected by wireless links to form an arbitrary topology. The routers, such as wireless mobile units, can move randomly and organize themselves arbitrarily as nodes in a network, similar to a packet radio network. The individual units require minimum configuration and their quick deployment can make ad-hoc networks suitable for emergency situations. For example, many MANET's are designed for military systems such as the JTRS (Joint Tactical Radio System) and other similar peer-to-peer or independent Basic Service Set Systems (IBSS).
TDMA technology is becoming more popular for use in these mobile ad-hoc network systems. In a TDMA ad-hoc network, channel access scheduling is a platform of the network structure. Some problems are encountered with distributed channel scheduling used in a multi-hop broadcast networks. As known to those skilled in the art, the optimum channel scheduling problem is equivalent to the graph coloring problem, which is a well known NP-complete problem, cited in numerous sources. Many prior art systems assume that the network topology is known and is not topology transparent.
There is a changing topology in a TDMA network. Before the network is formed, the topology cannot be learned. Without knowing the network topology, the nodes in the network must still find a way to communicate. Once the nodes learn about the transmit and receive schedules among neighbor nodes, these neighbor nodes may have moved away, disappeared, or new nodes may have moved in. The rate of resolving the scheduling must be fast and bandwidth efficient such that the network can be stabilized.
Additionally, there could be problems associated with distributed processing, overhead bandwidth, and network scalability. Centralized processing may not survive the changing topology, such as a partition in the network. Too much network bandwidth for scheduling could have a slow response to a change of topology. The scheduling is better off and scalable if it is highly distributed. A single point of failure could also be costly in terms of stability and speed of convergence.
A mobile ad-hoc network, with no fixed infrastructure, has also posed many challenges to the media access control (MAC) layer for multiple users. In a TDMA based ad-hoc network, it is sometimes difficult to assign non-interfering time slots to nodes and still allow spatial reuse of the bandwidth and time slots. Different nodes that are separated far enough would be able to reuse the bandwidth by using the same time slot.
A Carrier Sense Multiple Access (CSMA) based MAC design for an ad-hoc network is sometimes suitable for data transmission between mobile nodes. For example, 802.11 standards do not support voice or multimedia traffic with a desired quality of service because the transmission is contention based and asynchronous. The sharing of a radio resource often becomes indeterminist. To support voice and video streams, the system often requires bandwidth to be reserved. This could be accomplished in some TDMA based schemes by using non-interfering time slots, for example, channel scheduling.
A Code Division Multiple Access (CDMA) based system is bandwidth efficient in traditional cellular networks but it relies on adequate power control, maintained by the base station, for eliminating the near/far effect. In a mobile ad-hoc network environments, however, CDMA technology has encountered some difficulties.
There are many algorithms now being developed on the TDMA channel scheduling for mobile ad-hoc networks, but many of these proposed solutions are based on having an adequate knowledge of the network topologies. Such methods would require a substantial amount of overhead in the network, as the processor may be required to collect information about the entire network. In a highly mobile ad-hoc network, on the other hand, the information being collected may become stale at any moment. Therefore, the lightness in overhead and relative speed of generating the channel schedules become important factors in the design of the Media Access Control layer in mobile ad-hoc networks.